Over the past two decades many methods of synthesizing integral holograms have been investigated. These include transmission, image plane, projection and reflection integrals. Holograms of the integral type were first discussed in literature by Robert Pole of IBM in 1967. A number of improvements were advanced over the next several years. The most notable of these were developed by the Multiplex Company in 1969-1973; a group of individuals including Britton Zabka.
Reflection holograms are formed when the reference and object beams are on opposite sides of the holographic medium during the hologram forming process. Transmission holograms are formed when the reference and object beams are on the same side of the recording medium; the object beam on axis, the reference beam off axis. The reflection hologram has been variously referred to as a reflection integral, a composite reflection hologram, a holographic reflection stereogram, Bragg's angle hologram, a volume hologram, or an amplitude hologram.
As those trained in the art will appreciate, an integral hologram is made by running film images through an optical printer. In such a process, each frame of the film is projected through condensing optical elements which condenses the associated image down to a narrow vertical beam with a length corresponding to one of the sides of the finished hologram. This was initially done with a single double convex oil lens at Multiplex. This narrow beam is focused near the plane of holographic recording medium. The reference beam is brought in from the same side and an individual condensed slit or strip hologram is formed. Each succeeding frame of film used to form the hologram is made into a slit hologram, and sequentially laid down on the recording medium by slightly moving the recording medium between each exposure. When the resultant hologram composite is processed and then back illuminated with white light it reveals each individual strip hologram conveying an image which is two dimensional. However, each eye sees a slightly different slit and the viewer perceives a three dimensional image.
A conventional reflection hologram acts as a resonant reflective filter, both angle and wavelength specific, absorbing light while acting as wavelength filter, unlike transmission integrals that are capable of constructively diffracting light through the same areas with many wavelengths and that ordinarily do not filter light.
The Bragg or reflection hologram occurs because the object beam and reference beam approach on opposite sides of the recording medium. The superimposition of these two beams creates interference fringes which are extremely close together, caused by great angular separation at approximately one half of a wavelength of light. These fringes are so close and numerous that this hologram selectively reinforces one color wavelength at the same time playing back an image. The Bragg condition is only met for one wavelength which matches up to the precise spacing between wavelengths; all others are absorbed, as those trained in the art will appreciate.
Reflection integrals formed through conventional reflection synthesis produces a hologram with a severely reduced vertical viewing range, inadequate for displayed viewing.
Producing a high diffraction efficient reflection integral hologram with a normal vertical viewing range, that can also be displayed flat without distortion has been attempted by using screen techniques, multiple mechanized positioning of reference beams and other techniques. See, for instance, U.S. Pat. No. 4,411,489 (at column 12, lines 15-70, and column 13, lines 1-30) and U.S. Pat. No. 4,834,476 (at column 7, lines 31-42). However, the concept of dispersing the object and reference beams by conventional lenticular screens or similar dispersive transmissive or reflective optical elements adjacent to or in contact with the recording medium during hologram synthesis is not found appropriate for display applications, because it is beset with built-in limitations, as discussed below.
Even high quality optical elements are not free from irregularities in their surfaces, which may include fringe height variations, deviations, absorption, indexing, dispersion differences, or other errors and flaws. These optical elements, when put near or in contact with holographic emulsion that intercepts the recording light waves, reduces the imaging ability by refracting the recording light into slightly incorrect directions, which reduce the resultant holograms' diffraction efficiency, amplitude and clarity. The reflection integral wave synthesis requires optimization of Bragg's condition for best display applications.
Diffraction is reinforced by reflection by a series of accurately spaced planes formed by the object and reference beams which correspond to the wavelength and angular orientation. The angle at which this reinforcement occurs is Bragg's angle. When the wavelength of the object beam deviates even slightly, diffraction efficiency drops considerably, as those trained in the art will appreciate. Obviously very small imperfections whose refractive indexes differ only slightly from the rest of the optical elements have powerful effects in altering the image adversely. The slightest aberrations threaten the fidelity of the finished hologram. When dispersion elements diffuse wavefronts of coherent light, the phase undulations of the diffuse wavefront become amplitude variations which hinder the high clarity and contrast, and lower diffraction efficiency. The dispersion elements also rearrange beam amplitude and phase as they travel through or are reflected from these elements. The result is to limit or distort the hologram that is ultimately recorded. This disadvantage is compounded using a Fresnel reflection element because both reference and object rays will be dispersed.
U.S. Pat. No. 4,206,965, col. 12, lines 53-55 encourages mirrorized indexing of the resultant hologram to enhance brightness lost by use of dispersing elements. That mirrorizing is a well known technique is also evidenced by U.S. Pat. No. 3,758,649, noting col. 7, lines 7-11. Volume integral reflection holograms produce unwanted bands, called Newton Rings, caused by the interference synthesized from beams illuminating opposite sides of recording medium. These rings are properly indexed and eliminated by a black backing for the resultant hologram. Mirrorization, on the other hand, reflects them back to viewer, as those trained in the art will appreciate. Furthermore, mirrorization is unnecessary with a process, such as disclosed and claimed herein, which eliminates the need for dispersing elements.
U.S. Pat. No. 4,206,965, col. 8, lines 24-60, refers to mechanical multiple reference beam positioning per strip synthesis. For flat display purposes, this exhibits several disadvantages. The synthesis of reflection holograms are greatly prone to the effects of vibration by a higher factor than most any other types of holographic synthesis. Lengthy printing times are thus required because of the need to dampen factors introduced by moving the reference beam during hologram synthesis.